You can coast while doing intervals. It's just a different kind of interval training from active recovery. As a runner I did intervals that had jogging recoveries, and I have done intervals that had "lie panting and groaning on your back" recoveries. It depends on how intense, how long, and how many are the repeats.
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KazOct 3 '12 at 0:08

3 Answers
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The reason (and I'm no medical expert) is that it acts as a gradual cool-down period after the hard exercise. During any exercise it is good to cool-down by still moving and exerting less effort. Keeping the muscles moving helps remove lactic acid from the muscles (which contributes to cramping) and keeps the blood flowing. Also sudden changes between hard pedaling and stopping can make your blood pressure change more quickly. It is better to make gradual transitions.

From a bit of searching through training sites after reading that comment last night it seems that it is for the same reason that at the end of a 10k run you don't want to stop; if you do your muscles don't get a chance to remove toxins such as lactic acid that have built up.

The guidance for runners is to walk around and stretch gently - I'm guessing that the guidance on not coasting is for exactly the same reason - keep the muscles moving so as to aid in lactic acid dispersion and the removal of toxins built up during the exercise phase.

Of course, coasting for 30 seconds probably isn't going to hurt you, and any longer and you'll be rolling to a stop in most cases, so the "rule" is largely moot unless you're going down a steep hill.
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Daniel R HicksSep 14 '11 at 20:10

Yes, as others indicated, it is for the same reason that runners don't just stop and sit down between intervals.

I would add that it is also a matter of form. As much as they are about power, intervals are also about developing good form. If you punctuate them by totally slacking and using the bike as an easy chair that's somewhat counterproductive.